The invention relates to a process for calibrating a calorimetric flow indicator to a medium to be measured. The flow indicator comprises a sensor element having a temperature-dependent resistance and which heats up in a short period of time by means of electrical current heat and whose resistance variation (induced by cooling), which represents a measure for the flow velocity of a medium flowing by the sensor element, is measured. In the process, for a specified medium, the resistance value Ro at flow 0 and the resistance value R.sub.max at maximum flow define a preset range of measurement R.sub.max --Ro.
Media whose flow velocities are to be measured with a calorimetric flow indicator have varying heat capacities or varying thermal conductivities. Therefore, commercial calorimetric flow indicators have, for a specified medium, e.g. water or oil, a characteristic curve. An electric circuit is designed in such a manner that in the best case at flow 0, the lowest value on a measuring scale of a calorimetric flow indicator is assumed, and at maximum flow, the highest value on the measuring scale is assumed, and thus the range of measurement on the measuring scale is assumed. Thus all possible flow velocities are indicated within the measuring scale. A switching point, for example for indicating a flow or temperature threshold value of the measured medium in order to give a signal indicating that the threshold value has been exceeded, can be set within the range of measurement. Owing to these circumstances, a calorimetric flow indicator can be used optimally only for that medium for which is has been designed by the manufacturer. Yet the manufacturer often does not know for which medium the customer wants to use the flow indicator. If the device is adjusted and set by the manufacturer to, e.g. water, the customer, while measuring a different medium, may have an indication of a zero measured value even though there is already a flow.
It has therefore been proposed to provide such calorimetric flow indicators with means that allow a specific adaptation to the medium to be measured by adjusting the switching point that indicates a flow threshold value. However, this has the drawback that frequently the full measuring scale of the calorimetric flow indicator is not used up to the maximum flow velocity for different media to be measured, and thus, for example, only a portion of the indicator lights may light up at the maximum flow velocity in an LED display. At the same time, a portion of the measurement is "given away" or lost when the accuracy of the indicator is correspondingly reduced. Thus, it can happen that the switching point, at which a signal is supposed to be given that a threshold value of the flow velocity has been exceeded, is not within that portion of the measuring scale that is even scanned for the specific medium up to the maximum flow velocity.
In another known device a second potentiometer is provided that is rotated by hand until, at maximum flow velocity, the entire range of indication has been exploited, and thus, for example, in the case of an LED display, all indicating lamps light up. It assumes, however, not only that the start value, but also the final value, has been specified by hand. This is tedious manipulation that is to be avoided with the present invention.